The mechanism of veining and retrograde alteration of Alpine eclogites |
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Authors: | A.C. BARNICOAT |
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Affiliation: | Department of Geology, University College of Wales, Aberystwyth SY23 3DB, Wales |
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Abstract: | Abstract The introduction of externally derived fluids into rocks of the Zermatt–Saas zone of the Swiss Alps gave rise to the simultaneous formation of shear and hydraulic fractures. These fractures are now filled with albite-rich assemblages and surrounded by alteration halos up to c. 2 m wide. The alteration assemblages are zoned and an examination of reactions in P–T–a H2O space implies that the parageneses developed by the hydration of fluid-absent eclogites. A mechanical analysis of the veins (after Sibson, 1981) shows that P fluid/ P load must have been at least 0.96. Fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper, external reservoir, possibly tapped along thrust surface(s). Diffusive and capillary transport were insignificant. The fluids may have been derived from underlying metasediments that were dehydrating during the quasi-isothermal uplift of this part of the Alps, or they may have originated during the prograde mesoalpine metamorphism documented in the area. |
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Keywords: | Alps alteration fluid absent hydraulic fracture Zermatt–Saas zone |
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